Generally, large amounts of data being compressed and decompressed for numerous reasons, including transmission, storage, retrieval, and processing use at some stage means of variable-length coding, such as Huffman coding. Huffman coding was originally disclosed by D. A. Huffman in an article “A Method for the Construction of Minimum Redundancy Codes” Proc. IRE, 40: 1098, 1952. Huffman coding is a widely used technique for lossless data compression that achieves compact data representation by taking advantage of the statistical characteristics of the source. The Huffman code is a prefix-free variable-length code that assures that a code is uniquely decodable. In Huffman code, no codeword is the prefix of any other codeword. The run-length processed data are often subsequently coded by variable-length coding for further data compression.
Variable-length encoding allocates codes of different lengths to different input data according to the probability of occurrence of the input data, so that statistically, more frequent input codes are allocated shorter codes than the less frequent codes. The less frequent input codes are allocated longer codes. The allocation of codes may be done either statically or adaptively. For the static case, the same output code is provided for a given input datum, no matter what block of data is being processed. For the adaptive case, output codes are assigned to input data based on a statistical analysis of a particular input block or set of blocks of data, and possibly changes from block to block (or from a set of blocks to a set of blocks).
The combination of run-length coding and Huffman coding has been adopted in most compression/decompression standards. However, every standard has its own Huffman tables and run-length definitions. In principle, a programmable decoding table could be implemented by a random access memory (RAM) unit. However, it would require a 2^16 word RAM (if the longest length of a codeword is 16 bits) which would be very wasteful. It is much more efficient to use a content addressable memory (CAM) whose sizes are determined only by the number of codebook entries. However, if it were implemented by CAM, it would result in a circuit much larger and complicated than a circuit using only a small RAM. Thus there is a need for a simple and flexible variable-length decoder that can speedily and efficiently decode variable length codes of varying standards.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art through comparison of such systems with the present invention, as set forth in the remainder of the present application with reference to the drawings.